This invention relates to a resistance heating furnace, and more particularly, it relates to a high temperature heating furnace employing carbon materials as its resistance heating elements and furnace walls which enclose said resistance heating elements.
Conventional high temperature heating elements furnaces of the kind mentioned above are classified into two groups, viz., one shown in FIGS. 3 and 4 and called as a tubular direct heating high temperature furnace, and another shown in FIGS. 5 and 6 and called as a rod-type heating element high temperature heating furnace.
These high temperature heating furnaces have a metallic furnace frame 1 of a rectangular cross-section. There are provided at outer sides of the metallic furnace frame 1 water-cooled jackets 2. A protective case 11 made from carbon materials and having a circular or rectangular cross-section extends longitudinally along the axis of the furnace frame 1, and heating elements 5 are accommodated within the protective case 11.
Heat insulating materials 4 are contained in a space between the frame 1 and the protective case 11.
Since high temperature heating furnaces of the kinds mentioned above are employed for a heat treatment held at a high temperature between about 2,000.degree. and 3,000.degree. C., both of the aforementioned heating elements 5 and heat insulating materials 4 are made from carbon materials, that is, carbonaceous or graphite refractories.
Compared to the rod-type heating element high temperature heating furnace shown in FIGS. 5 and 6, electric supply structures for heating the heating element 5 (such as connecting terminals 6, insulators 7, terminals 8 of the heating element 5, and electric supply wires 10) in the above-mentioned tubular direct heating high temperature furnace become advantageously simple since the heating element 5 which is tubular and lies coaxially with the protective case 11, acts also as an inner furnace case, to both free ends of which common electric terminals are connected. However, on the other hand, it is disadvantegeous that a temperature distribution is not even along a longitudinal direction of the furnace. That is, since a temperature is highest at the middle portion of the furnace, and becomes lower towards the both ends of the furnace, a zone or area of an even temperature within the circular heating element which forms a furnace bed, is comparatively short. It is disadvantageous also in this kind of tubular direct heating high temperature furnaces that a heating loss of the heating element by a thermal conduction is large as the heating element constitutes, together with the protective case 11, furnace inlet and outlet openings directly exposed to an outer atmosphere.
The rod-type heating element high temperature heating furnace shown in FIGS. 5 and 6 belongs to a so-called indirect heating furnace, and has been developed so as to eliminate the aforementioned drawbacks or disadvantages accompanied to the tubular direct heating high temperature furnace which is illustrated in FIGS. 3 and 4 and belongs to a so-called direct heating furnace. In said rod-type heating element high temperature heating furnace, a longitudinal direction of the protective case 11 which is rectangular at its cross-section and accommodates therein a carbonaceous furnace bed 9, is supposedly divided to a plurality of sections, and to each of said sections at upper and lower positions thereof, there are provided carbonaceous heating rods 5 which extend transversely to the longitudinal direction of the protective case, whereby a zone or area of an even and common temperature can be enlarged and whereby a temperature distribution along the longitudinal direction of the furnace can be adjusted as desired. However, there are such drawbacks that heat dissipation is large since many through holes have to be formed in the protective case 11, carbonaceous heat insulating materials 4, and metallic furnace frame 1 in order to supply electric currents independently to each of the heating rods 5. In FIGS. 5 and 6, those members or parts which are identical to those of FIGS. 3 and 4, are represented by same numerals.
While as described above, conventional high temperature heating furnaces have their respective drawbacks, characteristic features common to them are that they employ unanimously the protective case 11 which extends along a longitudinal direction of the furnace frame and is made from carbon materials. The primary object of such employment of the protective case 11 either of a tubular or rectangular shape is to insulate from the heating element 5 the carbonaceous thermal insulating material 4 which are made of carbon or graphite granules or powders, carbon fibers (including those molded to a felt, or in bulk or blocks), or porous carbon materials.
The secondary object is to have the thermal insulating materials 4 keep a desired configuration so that they can accomodate a space in which a furnace bed is provided, when they are made of carbon or graphite granules or powders.